The cleaning and surface treatment of tinplate is frequently conducted by a spray process. For example, the surface treatment equipment for tinplate DI can is generally called a washer. Molded DI can is inverted and continuously treated in the washer with a cleaning bath and a surface treatment bath. Existing washers normally use 6 steps (pre-cleaning, cleaning, water wash, surface treatment, water wash, and wash with de-ionized water), and treatment is conducted entirely by spraying.
Compositions of phosphate ion, tin ion, and oxidizing agent are already known as surface treatment baths for tinplate DI can. As discussed by the present inventors in Nihon Parkerizing Giho, 89, No. 2, page 6, the mechanism of conversion film formation by these components consists of tin and iron elution reactions (anodic reactions) and the precipitation of insoluble phosphate salts (cathodic reaction).
Furthermore, in Japanese Patent Application Laid Open [Kokai or Unexamined] Number Hei 1-100281 [100,281/1989]), there has already been proposed a composition for the purpose of inverting the tin-iron potential in the conversion bath, i.e., the tin region becomes the anode and the iron region becomes the cathode. This particular invention consists of a conversion coating bath for the treatment of metal surfaces. This bath has a pH of 2 to 6 and contains 1 to 50 grams per liter (hereinafter often abbreviated "g/L") of phosphate ions, 0.2 to 20.0 g/L of oxyacid ions, 0.01 to 2.0 g/L of tin ion, and 0.01 to 5.0 g/L of condensed phosphate ions. Treatment with this conversion treatment bath forms a highly corrosion-resistant, highly paint-adherent phosphate film on the surface of tinplate DI can. The oxyacid ion is an oxidizing agent that functions to oxidatively remove the hydrogen that is produced by the anodic reactions.
When the aforesaid invention is practiced on a continuous basis, it is in fact capable of initially providing an excellent surface treatment. However, it has been found that the referenced invention gradually generates a phosphate salt sludge, which is produced by the reaction of the phosphate ions present in the bath with the tin ions and iron ions that elute from the tinplate. It has also been determined that iron ions elute from the tinplate in the divalent state; that gradual oxidation by the oxidizing agent (oxyacid ion, etc.) produces the trivalent state in the surface treatment bath at a level of approximately 0.05 g/L; and that this is the cause of sludge production.
This sludge can cause problems by adhering to the tinplate surface and degrading the paint adherence. In addition, the sludge can clog the piping and nozzles of the spray equipment and can thereby prevent a high quality surface treatment. This has necessitated the implementation of periodic maintenance in order to clean the piping and nozzles of the spray equipment and has resulted in unstable quality characteristics. Since productivity enhancements and improvements in quality stability have recently become critical issues, a surface treatment bath is desired that carries a reduced cleaning burden and that offers stable quality characteristics, i.e., that is free of sludge production in the bath even during continuous service.